This invention relates to an apparatus and method for control of extrusion of a plastic stock material, and more particularly to an apparatus and method of extrusion control providing improved extrusion shape uniformity through the control of viscosity.
As is well known in the industry, a plastic extruder is frequently used to process plastic whenever one dimension is continuous, for example garden hose, plastic rod, plastic sheet, film, etc. A plastic extruder consists of a hollow, heated metal barrel with a screw turning inside of it such that plastic pellets or powder fed into one end of the barrel are melted, mixed and conveyed to the other end. By the time the plastic reaches the output end, considerable pressure has been built up, so that molten plastic, called melt, can be forced through a die which shapes it in two dimensions. Extrusion is continuous, and therefore the length dimension is continuous, as in the previous cited example of garden hose, rod, sheet, film, etc.
The barrel of the extruder is divided into a number of zones, usually 3 to 6. The pitch and depth of the screw varies in these zones so that the different functions of conveying the plastic pellets (or powder), melting the plastic, conveying the plastic in the melt, mixing thoroughly, homogenizing the temperature, and building up the pressure for the die can be accomplished most expediously. To accomplish their individual functions best, the zones are each held at a particular temperature which is dictated by the design of the screw. Each zone has an individual temperature altering means in the form of a heater or combined heater and cooler clamped to the outside of the barrel and one (or in some cases two) temperature sensors connected to a temperature controller whose function it is to control the temperature of each individual zone at the value dictated by the screw design, in spite of any changes that may occur in the frictional heat generated by the rubbing of the plastic against the barrel and the screw.
It should be noted that in such an extruder as this, most of the heat required to raise the temperature of the plastic to the melting point, to melt it, and then to raise the temperature of the melt to the temperature at which it is desired to force it through the die, comes from the frictional heat generated by the motion of the screw as it is driven by the motor. The temperature altering means clamped to the outside of the barrel are necessary to make up for the fact that the temperature of each individual zone might not be at the desired value for that zone if frictional heat alone were used. Therefore heat is added or subtracted at the individual zones to bring the temperature of each zone precisely to the desired point.
It is not only the temperature of the plastic at each of the zones of the screw that is of importance. For after the plastic leaves the screw, it is to be forced through a die to form the desired shape of the final product. The various properties of the plastic as it reaches the die are of major importance in determining the quantity and quality of the extrudate.
Pressure at the die is one of the most important factors. For, other things being equal, the higher the pressure at the die, the more plastic will be forced through, and the more product will be produced in any given time.
It is desirable to have a constant rate of extrusion, in order that the cross-sectional dimensions of the extrudate will be constant along its length. Process controllers which maintain the pressure at the die constant are thus in common use. One such is the Harrel CP-660 panel mount Digipanel system, commercially available and described in the technical data sheet TDS-251 of Harrel, Incorporated. These controllers sense the pressure at the die and adjust the speed of the screw in such a way as to hold the pressure constant. When such a pressure controller changes the speed of the screw, the frictional heat generated by the screw will change, and this will in turn change the temperature at each of the barrel zones described above. However, each of these zones has associated with it individual temperature control, and this control acts to bring the zone back to its setpoint, which is the preset desired value of temperature. This inventor's U.S. Pat. No. 4,272,466 describes controllers that control the temperature in the individual zones.
Another quality of the plastic melt at the die which is of importance is the temperature. Temperature at the die is important for two reasons. First, many plastics degrade, and this degradation will increase with increasing temperature. For such materials as polyvinyl chloride, it is important that the temperature of the melt be held as low as possible to minimize degradation of the plastic. The second reason that temperature at the die is of consequence is that it affects viscosity. For most materials, the higher the temperature the lower the viscosity, and hence the more that will be forced through the die per unit time at a given die pressure.
Melt temperature process controls are in common use, again as described in the aforementioned technical data sheet and throughout the literature. These measure the melt temperature at or near the die by means of a temperature probe actually inserted into the melt stream downstream of the screw. The melt temperature control then adjusts the setpoint values of the barrel zone temperatures up or down as required to hold the melt temperature constant. If the characteristic of the plastic and of the rest of the extrusion process remain the same, holding the temperature of the plastic at the die constant will hold the viscosity of the plastic constant. If the die pressure is also held constant, constant viscosity will also mean constant throughput and hence constant dimensions of the product in the length dimension.
Melt temperature control is useful, but unfortunately, holding the temperature at the die constant does not always hold the viscosity constant there. Temperature does most certainly affect viscosity, but so do many other factors, e.g. the melt index of the plastic, the degree to which it is agitated, the molecular weight, etc. The most common variation in the relationship between temperature and viscosity is due to the use of regrind material. Whenever plastic is extruded, some scrap is inevitable, for example, because of off-tolerance operation, or product wasted during start up and shutdown. It is customary to grind up this scrap and put it back in the hopper to reuse it. Sometimes plastic may actually go through the extruder three of four times in this manner.
In many plastics the physical properties are significantly changed once it has gone through the extruder. The physical properties are simply not the same between regrind and virgin plastic, and one of the most significant changes is in the viscosity which results from a given set of processing conditions. This means that the combination of a melt pressure controller and a melt temperature controller will not result in constant throughput and hence extrudate dimensions if the plastic is changed from virgin material to a blend of virgin material and regrind or to pure regrind, for example.
The problem of variation in throughput with varying melt viscosity can be solved by the use of a gear pump operating in tandem with the extruder as described in the current inventor's U.S. Pat. No. 4,209,476. A gear pump has closely meshed, counter-revolving gears in a close fitting housing. If material is introduced into one side of this set of gears, the amount of plastic that is transmitted to the other side as the gears are turned can be made to be virtually independent of the viscosity and dependent almost exclusively on the speed that the gears are turned and the volume of the gear teeth.
Stabilizing throughput with a gear pump is a tremendous advance, but as so often happens, when one improvement is made the desirability of another appears. Use of the gear pump has compensated for one effect of varying melt viscosity, i.e. the variations in throughput that it causes. It has not, however, eliminated the variations in viscosity itself. As the ratio of virgin material to regrind is changed, for example, we will still get the same quantity of plastic through the die, but the viscosity of the extrudate as it exits will be different. This difference in extrudate viscosity is very undesirable. For the shape of the extrudate can be affected markedly by the viscosity as the plastic enters the water bath or other cooling means upon leaving the extruder. It is particularly undesirable where post-extrusion forming operations are to be performed, as in blow molding or blown film. It would be very desirable if not only the throughput, but also the viscosity of the melt could be stabilized. It is the purpose of the present inventions to do this.